An unprecedented federal review of old criminal cases has uncovered as many as 27 death penalty convictions in which FBI forensic experts may have mistakenly linked defendants to crimes with exaggerated scientific testimony, U.S. officials said.

The review led to an 11th-hour stay of execution in Mississippi in May.

It is not known how many of the cases involve errors, how many led to wrongful convictions or how many mistakes may now jeopardize valid convictions. Those questions will be explored as the review continues.